Can you override the default formatter for f-strings?

Question

Reading through PEP 3101, it discusses how to subclass string.Formatter to define your own formats, but then you use it via myformatter.format(string). Is there a way to just make it work with f-strings?

E.g. I'm looking to do something like f"height = {height:.2f}" but I want my own float formatter that handles certain special cases.

Answer 1

As I mentioned in comments F-strings are actually processed at compile time, not runtime. you can't override the default f-string formatter is due to how Python implements f-strings at a fundamental level.

For example if you run

f"height = {height:.2f}"

It effectively converts

"height = {}".format(format(height, '.2f'))

conversion happens during compilation, there's no way to "hook into" or modify this process at runtime.

What you can do is.

use decimal Module for Precision Control.

Reference - https://realpython.com/how-to-python-f-string-format-float/

from decimal import Decimal

value = Decimal('1.23456789')
print(f"Value: {value:.2f}")

Output:

Value: 1.23

Decimal implements the format method and handles the standard format specifiers like .2f.

Side Note:

We aren't really "overriding" the default formatter here - it's implementing the existing format specification protocol.

As your goal is to handle special cases for float formatting, using Decimal could solve your problem...